1. Technical Field
The present Invention relates to an apparatus and method for designing a photomask for use with production of a semiconductor device, and it particular relates to the apparatus and method for designing a phase-shifting masks to improve the resolution of microlithography for a miniaturized pattern for semiconductor device production.
2. Background Art
A photomask on which an original mask pattern of a VLSI is drawn is irradiated by an incident ray which is partially coherent. For convenience, tile photomask will be simply referred to as a mask hereinafter. Then, the pattern on the mask is transcribed on to a semiconductor wafer so as to execute photo lithography. For a projection lithography system realizing the photo lithography, there is required a further miniaturized pattern to be copied on to the wafer.
How much miniaturized the pattern can be fabricated is expressed by a degree of resolution therefor. The resolution is evaluated by how accurately two adjacent portions constituted by light and shade portions on a semiconductor substrate can be separated by utilizing, say, a mask pattern whose light and shade are periodically changed. In order to improve the resolution, there is known a method where a phase difference is given to the incident ray which passes a pair of adjacent clear regions on the mask.
As for the method where the phase difference is given to the incident ray which passes a pair of adjacent clear regions on the mask, such a method is conventionally known and discussed in a paper entitled "Improving Resolution in Photolithography with a Phase-Shifting Mask" written by Mark D. Levenson et al. (IEEE Transaction Electron Devices, Vol ED-29 No. 12, 1982, page 1828).
FIG. 1 shows a Levenson type phase shift mask suggested by the above paper. In the same figure, on mask substrate 1 there are formed a plurality of light-shielding portions (opaque regions) 2 serving as the original images for tile pattern and clear region 1, and there is provided phase shifter 3 which supplies a phase difference to the incident ray passing through the clear region; phase shifter is made of transparent material. Phase shifter 3 is arranged to be disposed on either side of a pair of the adjacent clear regions. The phase shifter 3 may be simply referred to as a shifter hereinafter.
The shifter need satisfy a condition of EQU d=.lambda./{2(n-1)},
where d is a film thickness, n an index of refraction and .lambda. a wavelength of the incident ray. The ray which passes through the shifter differs from other transmission rays that didn't pass through the shifter, so that the light intensity of the ray in a pattern boundary on the semiconductor substrate becomes zero. As a result thereof, adjacent bright regions are separated, thus improving the resolution therefor.
In connection with the paper discussing the Levenson type phase shifting mask and an automatic designing apparatus therefor, there is a paper entitled "Automatic Pattern Generation System for Phase Shifting Mask" authored by Noboru Nomura et al. (Symposium on VLSI Technology, JSAP CAT No.AP911210, pp. 95-96, Oiso, Japan, May 1991). In the paper written by Nomura et al, there is discussed an automatic designing apparatus for a Levenson-type phase shifting mask. In Nomura et al's method, the shifters are automatically arranged in a unit of a cell constituted by a plurality of opaque and clear regions, as shown in FIG. 1, then, when a portion in which it is impossible to arrange the phase shifter properly is found a system of Nomura gives a warning and aborts.
Moreover, when there is no place where the shifter arrangement is impossible, the verification is done to determine whether or not the arrangement is correct in the vicinity of the boundary for each cell after the shifter arrangement is completed within the cell. If an error is found in the shifter arrangement in the vicinity of the boundary for the adjacent cells, the shifter in all clear regions belonging to one of the pair of cells is reversed.
Besides the above two papers, there is another paper entitled "Investigating Phase-shifting Mask Layout Issues Using a CAD Toolkit" written by Andrew R. Neureuther et al. (IEDM Technical Digest, pp 705-708, 1991). This paper discusses an automatic designing system where a circuit designer determines a shrink factor for a given design pattern, the shifter is automatically assigned against the shrink pattern, and, if there is a portion where the shifter assignment conflicts, such portion is notified.
However, in any of the known automatic designing systems, an automatic verification function for the masks In which the shifter arrangement therefor is already entirely completed is not provided. Thereby, in the case where the shifter arrangement is performed by hand, the shifter arrangement can not be determined whether it is correct or not.
Moreover, automatic verification and automatic assigning functions against a partially arranged mask are not provided. Referring to FIG. 2, when considering to automatically assign the shifter for the mask where there are formed the hatched clear regions lingering between two neighboring regions 18, 19, an inconvenience illustrated below is caused.
Namely, referring to FIG. 2, suppose that, first, the shifter is assigned to the region 18. The phase of the clear region which extends to region 19 is determined. Then, when the automatic shifter arrangement is invoked for region 19, there is already a clear region where the shifter arrangement is partially completed. In that case, ideally speaking, it is necessary to verify automatically the shifter arrangement for the region where the shifter arrangement is completed, and to assign automatically the shifter for the region where the shifter arrangement is not yet performed.
Moreover, the above-mentioned automatic verification and automatic arrangement are also necessary for a case where there is a spot in which the circuit designer wishes to arrange a shifter with a priority therefor, and after the shifter is manually assigned to the spot, the rest of the clear region shall be automatically shifter-assigned.
Accordingly, in the conventional technique, where the shifter is assigned to one of a pair of neighboring clear regions, is improved the resolution according to a principle of the Levenson-type phase shifting mask. However, in this technique, how closely the clear regions on the mask are situated is not verified, so that the shifter is assigned regardless of the distance between the clear regions. Thereby, there causes a drawback where the shifter is assigned to the clear regions in which the phase difference need not be given.
Moreover, since the conventional automatic designing system is not equipped with the automatic verification capability, there is caused a problem where whether or not the arrangement is correct can not be verified against the mask in which the shifter therefor is already assigned. Moreover, in the case where there exists a spot in which the shifter arrangement is impossible, the warning is given so as to abort the system or such an impossible spot is indicated; however, any information regarding how to correct such a deficiency is not indicated in the conventional methods.
Moreover, in the conventional technique represented by Andrew R. Neureuther et al, when a layout data containing hierarchical layers is input, a method where the shifter Is arranged while maintaining the hierarchy is not considered. Therefore, in order to arrange the shifter and verify the arrangement, such the arrangement and the verification therefor are executed only after hierarchy is developed. Thus, the layout data is increased based on an increase in the recursions of the pattern, so that there is caused a problem where a memory therefor overflows or data processing time is undesirably increased.
Moreover, in the conventional technique well represented by Noboru Nomura et al, though there is equipped a function for automatically verifying an intercell phase, it is not at all clear that the shifter arrangement shall be initialized from which particular cell in case there exist a plurality of cells constituting the hierarchy. With reference to FIG. 3, cell A is composed of lower cell B and lower cell C. If a layout for cell B is modified after cell A is shifter-assigned, and the shifter arrangement is executed upon cell A again, the shifter arrangements are executed anew against cells A, B and C since in the conventional technique there is not provided memory means by which cell C is already shifter-assigned. Accordingly, the conventional technique is very inefficient.
In the Noboru Nomura et al's conventional technique, a dynamic random access memory (DRAM) is designed by utilizing the automatic Levenson type phase-shifting mask designing apparatus. In the event the phase of the light passing through a pattern is automatically determined, first of all, an arbitrary pattern is set to be at 0.degree.--shifted (normal transmission). After such an initialization, the original pattern whose phase has not yet been attributed is determined by the phase of a neighboring pattern in a manner that the neighboring pattern is chosen so that its side length facing the original patterns is longer than any other neighboring patterns whose phase have already been determined. Thereafter, the phase of the original pattern is set to be opposite to that of the neighboring pattern then chosen. When plural neighboring patterns with different phases share an identical facing side length, the apparatus aborts and gives warning.
In the above Nomura's technique, a peripheral area for certain patterns is not defined as which particular area is designated, so that a shifter assignment therefor may result in being impossible in spite of the fact that a layout therefor presents possible for the shifter assignment, after all. For example, suppose that there is given a layout data shown in FIG. 4A, where distance r1 is such that the phase of transmitting light is of opposite phase and a resolution therefor is possible, whereas the phase is identical and the resolution therefore is impossible. Suppose that distance r2 is such that the resolution is possible regardless of the phase being identical or opposite. Consider the execution of the shifter arrangement under such a condition, since there is given no particular instruction as to which particular order the phase of patterns is to be determined, patters X, Y, Z are determined the phase therefor in this order. A phase of X is determined, so that the phase of Y can be opposite to that of X determined, however, since side lengths of X facing both X and Y are equal and, worst, X and Y are opposite in phase to each other, an appropriate phase can not be given to Z. Notwithstanding, it is possible to practically determine such a phase as shown in FIG. 4B.
Furthermore, according to Japan Patent Laid Open No. 62-50811 (referred to as document S62 hereinafter), a phase shifter is provided in at least one of neighboring opening portions on the photomask, so that the phase difference is given to illuminating light passing through the two opening portions. Referring to FIGS. 5A through 5D, there is described how the resolution is improved by utilizing the above technique. Opening portion 54 has phase shifter 55 which gives a phase shift by 180.degree.. An at-least partially coherent light that passed through opening portion 53 and opening portion 54, causes an Interference on the wafer where the phase difference therebetween is 180.degree., as shown in FIG. 5C. On the other hand, absolute values of amplitudes, at an intermediate position of opening patterns on the wafer, for the light that transmitted through opening portion 53 and opening portion 54 are identical to each other. Therefore, intensity of synthesized light becomes 0 at the intermediate position of the opening portion on the wafer. For the reason set forth above, the resolution is improved in the middle portion of pattern.
In document S62, there is described an embodiment illustrated in FIG. 6. The mask has two-dimensional patterns in which there are three neighboring openings, and the phase-shifting member is not provided to the first opening 61, whereas the phase-shifting members are given to the second opening 62 at 90.degree. and the third opening 63 at 180.degree., so as to hopefully improve the resolution of the entire pattern. In this case, there are utilized 0.degree., 90.degree. and 180.degree. as phase-shifting degrees.
Besides document S62, in Japan Patent Laid Open No. 4-221954 (referred to as document H4 hereinafter), there is also shown another example dealing with the three neighboring openings. In document H4, referring to FIG. 7, the shifter member is not provided in opening 71 (0.degree.), while the shifter members are provided to the second opening 72 at 120.degree. and the third opening 73 at 240.degree.. Document H4 discusses a method where the phase difference between arbitrary two openings are set to 120.degree..
Referring again to FIGS. 5A-5D, the Levenson type phase-shifting method is very effective for a simple repetition such as one for line and space. However, in a case where there exists a spot of more than three neighboring opening patterns in a mask having a general two-dimensional pattern, there may be generated a spot where the phases of the light passing through the neighboring portions are identical, so that the resolution performance thereof is approximately same with the conventional lithography technique which does not employ the phase shifters. Hereinafter, the opening region is called a contradictory spot where the phases become identical no matter how the shifter is arranged in the Levenson-type phase-shifting method.
In order to obtain as great a shrink factor as possible, the number of contradictory spots must be minimized. In other words, when the Levenson-type phase-shifting method is applied to electronic devices such as a DRAM, the method is effective to a portion that requires many simply repeated patterns such as a cell array portion; however, in a portion having a sense amplifier or the like which are extended peripherally from the cell array portion, many complicated patterns are arranged. When the Levenson-type phase-shifting method is adopted to the complicated patterns, the patterns need be rewritten in a manner to minimize a portion exhibiting the identical phase, thus creating a very difficult task to perform. These restraints in the course of circuit designing become a serious burden in adopting the Levenson-type phase-shifting method to the electronic devices.
As described in document S62 (see FIG. 6) and document H4 (see FIG. 7), the resolution of the opening patterns that are mutually adjacent to each other are improved by utilizing three different phases; the resolution for transcription patterns are improved in a partial region or an entire region of the wafer including the mutually neighboring three opening portions as the transcription patterns. It is noted here that the Levenson-type phase-shifting mask is best in terms of resolution power where the phase difference is 180.degree.. Now, none of the above documents discuss about a particular method by which the shrink factor for an entire exposure mask is increased in the course of assigning phase shifters of more than three different phases in view of the above-mentioned fact that the Levenson-type phase-shifting mask is best in terms of resolution power with the phase difference being 180.degree.. In particular, in an embodiment shown in document S62, phases assigned are 0.degree., 180.degree. and 90.degree. in the case where three openings are mutually adjacent. There is not described any particular design rule as to how to assign such phase shifters to each opening, in document S62. Moreover, document S62 does not discuss on a definite phase-shifter assigning method which can cope with the case shown in FIG. 7.